The present invention is directed to integrated circuits. More particularly, the invention provides a system and method with multi-function protection. Merely by way of example, the invention has been applied to driving one or more cold-cathode fluorescent lamps, and/or one or more external-electrode fluorescent lamps. But it would be recognized that the invention has a much broader range of applicability.
The cold-cathode fluorescent lamp (CCFL) and external-electrode fluorescent lamp (EEFL) have been widely used to provide backlight for a liquid crystal display (LCD) module. The CCFL and EEFL often each require a high alternate current (AC) voltage such as 2 kV for ignition and normal operation. Such a high AC voltage can be provided by a CCFL driver system or an EEFL driver system. The CCFL driver system and the EEFL driver system each receive a low direct current (DC) voltage and convert the low DC voltage to the high AC voltage.
FIG. 1 is a simplified conventional driver system for CCFL and/or EEFL. The driver system 100 includes a control subsystem 110 and an AC power supply subsystem 120. The control subsystem 110 receives a power supply voltage VDDA and certain control signals. The control signals include an enabling (ENA) signal and a dimming (DIM) signal. In response, the control subsystem 110 outputs gate drive signals to the AC power supply subsystem 120. The AC power supply subsystem 120 includes MOSFET transistors and power transformers, and receives a low DC voltage VIN. The MOSFET transistors convert the low DC voltage VIN to a low AC voltage in response to the gate drive signals. The low AC voltage is boosted to a high AC voltage VOUT by the power transformers, and the high AC voltage VOUT is sent to drive a system 190. The system 190 includes CCFLs and/or EEFLs. The system 190 provides a current and voltage feedback to the control subsystem 110.
As discussed above, the power transformers can boost the AC voltage. The increase in AC voltage is often accomplished by a high turn ratio between the secondary winding and the primary winding. The secondary winding usually is formed by a wire having a small diameter such as 0.05 mm. The wire can easily be damaged by bending in the manufacturing process. For example, a breakpoint may exist at the winding terminal that is connected to pins in the transformer bobbin. If the gap at the breakpoint is small, the high AC voltage can jump through the gap by arcing and still drive the system 190 including CCFLs and/or EEFLs. But the arcing process can produce a large amount of heat and even a visible fire. Under these conditions, the driver system 100 should be turned off to prevent any accidents.
FIG. 2 is a simplified conventional system for detecting breakpoint in transformer secondary winding. The secondary winding of a transformer TI includes pins 5 and 6. The pin 6 is biased to the low DC voltage VIN that is different from the ground voltage. Additionally, the DC voltage at the pin 5 is received by a high impedance voltage divider. As shown in FIG. 2, the voltage divider includes resistors R1 and R2 and outputs a voltage VDIV to a transistor Q1. If no breakpoint exists in the secondary winding, the voltage VDIV would be equal to a fraction of VIN. As a result, the transistor Q1 is turned on, and the control subsystem 110 is enabled. If a breakpoint exists in the secondary winding, the voltage VDIV would be equal to zero. As a result, the transistor Q1 is turned off, and the control subsystem 110 is disabled. The driver system 100 for CCFL and/or EEFL is thus protected. But the system as shown in FIG. 2 often cannot effectively detect breakpoints for multiple transformers.
Hence it is highly desirable to improve protection techniques for CCFL driver system and EEFL driver system.